OPEN CYCLE OTEC
Open cycle systems use warm seawater as the working fluid. Warm
surface water evaporates under a partial vacuum. Low-pressure steam, which turns a turbine, is produced. The
turbine drives an electrical generator. The steam is condensed, either through mixing and direct contact with
the cold seawater (a more energy efficient method), or through chilling in a "surface" heat exchanger through which the
cold seawater is pumped. The latter method produces fresh water, giving many benefits.
CLOSED CYCLE OTEC
Warm surface seawater gets pumped into a heat exchanger where a low-boiling point "working" fluid (commonly ammonia)
is vaporized. Expanding vapor causes a turbine to turn and drive an electrical generator. Cold deep seawater,
which is pumped through a second heat exchanger, causes the vapor to condense back to a liquid which can then be returned
to the first heat exchanger.
Mini-OTEC, a 1979 project of the State of Hawaii, Lockheed Missiles & Space Co., Hawaiian Dredging and Construction
Co. and other industrial partners, succeeded at the first successful at-sea production of net electrical power from
OTEC. Ammonia vaporized in titanium plate heat exchangers helped to turn a turbine-generator, which gave a gross
power output of 50 kW. After other processes, only 10 kW available for electrical uses on the Mini-OTEC barge that
was moored about 1.5 miles (2.4 km) offshore of Keahole Point. This illustrates greater need for research on energy efficiency.